Electrode material, electrode, and battery

ABSTRACT

An electrode includes a plant-derived porous carbon material having an ability to catalyze oxygen reduction.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Japanese Priority PatentApplication JP 2013-020215 filed Feb. 5, 2013, the entire contents ofwhich are incorporated herein by reference.

BACKGROUND

The present disclosure relates to an electrode material, an electrode,and a battery.

Electrodes having an ability to catalyze oxygen reduction (hereafter,may be referred to as “oxygen reduction electrode”) are used in varioustypes of batteries, electrodes, and sensors such as:

(A) enzymatic biofuel cells that use organic matter such as sugar,alcohol, or cellulose as a fuel;

(B) microbial fuel cells used in wastewater treatment or sludgetreatment, in which a reaction is caused using organic matter containedin the wastewater or sludge to decompose the organic matter;

(C) metal-air batteries that include a negative electrode with anegative-electrode-active material including a metal and an alloyedmaterial;

(D) various types of fuel cells such as a polymer electrolyte fuel cell,a phosphoric-acid fuel cell, a molten carbonate fuel cell, a solid-oxidefuel cell, an alkaline fuel cell, and a direct-methanol fuel cell;

(E) gas diffusion electrodes for oxygen reduction used in brineelectrolysis; and

(F) electrochemical sensors for detecting oxygen.

An existing oxygen reduction catalyst, which constitutes the oxygenreduction electrode, having a remarkable ability to catalyze oxygenreduction is Platinum (Pt). However, because of the high price ofplatinum, there has been a strong demand for a replacement catalyst forplatinum.

A metal-air battery including a negative electrode containing zinc,aluminium, or magnesium and a positive electrode containing at least oneoxygen reduction catalyst is known from, for example, JapaneseUnexamined Patent Application Publication (Translation of PCTApplication) No. 2012-517075. The metal-air battery disclosed inJapanese Unexamined Patent Application Publication (Translation of PCTApplication) No. 2012-517075 includes an oxygen reduction catalystcontaining a noble metal or the like, and the oxygen reduction catalystis supported on carbon black, graphite, charcoal, activated carbon, orthe like.

SUMMARY

In Japanese Unexamined Patent Application Publication (Translation ofPCT Application) No. 2012-517075, carbon black, graphite, charcoal, andactivated carbon are referred to as a material on which an oxygenreduction catalyst is supported. However, there is no mention inJapanese Unexamined Patent Application Publication (Translation of PCTApplication) No. 2012-517075 that these materials themselves may serveas an oxygen reduction catalyst.

For example, since enzymatic biofuel cells and microbial fuel cellsdesire a pH-neutral operating condition, their oxygen reductionelectrodes (oxygen reduction catalysts) have to exhibit an ability tocatalyze oxygen reduction under a pH-neutral condition. However, typesof an oxygen reduction catalyst that exhibits the ability to catalyzeoxygen reduction under a pH-neutral condition are limited, and theoxygen reduction catalysts disclosed in Japanese Unexamined PatentApplication Publication (Translation of PCT Application) No. 2012-517075have a poor ability to catalyze oxygen reduction under a pH-neutralcondition. An existing example of an oxygen reduction catalyst having anability to catalyze oxygen reduction under a pH-neutral condition is anenzyme “bilirubin oxidase” (see, 221^(st) ECS Meeting, 2012 TheElectrochemical Society, Abstract #1437, Kano Kenji, “Significance ofCarbon Electrode Materials to Improve the Performance of DET-typeFructose/O₂ Biofuel cells”). However, a battery using this enzyme has alow electric current density, and the maximum electric current densitythat has been reported to date is about 40 mA/cm². In addition, one ofthe most serious disadvantages of enzymes are their low stability.

Accordingly, it is desirable to provide an electrode material capable ofserving under a pH-neutral condition, an electrode including theelectrode material, and a battery including the electrode.

According to an embodiment of the present disclosure, there is providedan electrode including a plant-derived porous carbon material having anability to catalyze oxygen reduction.

According to an embodiment of the present disclosure, there is providedan electrode material including a plant-derived porous carbon materialhaving an ability to catalyze oxygen reduction.

According to an embodiment of the present disclosure, there is provideda battery including a positive electrode including a plant-derivedporous carbon material having an ability to catalyze oxygen reduction.

The electrode material, the electrode, and the positive electrode of thebattery according to the embodiment of the present disclosure include aplant-derived porous carbon material having an ability to catalyzeoxygen reduction and therefore sufficiently exhibit an oxygen reductionability under a pH-neutral condition. Furthermore, since the electrodematerial, the electrode, and the positive electrode of the batteryaccording to the embodiment of the present disclosure do not useenzymes, there are fewer limitations on the operating environment of theelectrode and battery, which depends on the structure of the battery.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the results of evaluation of the oxygenreduction abilities of test oxygen reduction electrodes prepared inExample 1 and Comparative Examples 1A to 1D;

FIG. 2 is a graph showing the results of evaluation of the oxygenreduction ability of a platinum electrode, which is shown as a referenceexample;

FIGS. 3A and 3B each show the results of measurement of the cumulativepore volumes of a plant-derived porous carbon material used in Example 1and a material used in Comparative Example 1A;

FIG. 4 is a graph showing the results of evaluation of the oxygenreduction ability of a test oxygen reduction electrode prepared inExample 2;

FIG. 5 is a graph showing the results of evaluation of the oxygenreduction ability of the test oxygen reduction electrode of Example 2;

FIG. 6 is a graph showing the relationship between the corrosion rate ofaluminium and pH;

FIG. 7 is a graph showing the results of evaluation of the oxygenreduction ability of an electrode prepared in Example 3;

FIGS. 8A and 8B are schematic cross-sectional views of a batteryprepared in Example 3;

FIG. 9 is a graph showing the cell voltages of the battery of Example 3,which were recorded while the battery was controlled so that apredetermined load was applied to the battery and a certain amount ofcurrent flowed;

FIG. 10 is a schematic cross-sectional view of a variation of thebattery of Example 3;

FIG. 11A is a cross-sectional view of a cartridge of the battery ofExample 3 shown in FIG. 10, which has not yet been used;

FIG. 11B is a cross-sectional view of a cartridge of the battery ofExample 3 shown in FIG. 10, which has been used;

FIG. 11C is a schematic cross-sectional view of the battery of Example3, which has been used;

FIG. 12 is a schematic cross-sectional view of the battery of Example 3for explaining a method for replacing the cartridge of the battery ofExample 3;

FIG. 13 is a schematic cross-sectional view of a battery prepared inExample 4;

FIG. 14 is a graph showing a cell voltage of the battery of Example 4,which was recorded while the battery was controlled so that apredetermined load was applied to the battery and a certain amount ofcurrent flowed;

FIG. 15 is a schematic cross-sectional view of a variation of thebattery of Example 4; and

FIG. 16 is a schematic cross-sectional view of another variation of thebattery of Example 4.

DETAILED DESCRIPTION OF EMBODIMENTS

Hereafter, the present disclosure is described on the basis of thefollowing examples with reference to the attached drawings. However, thepresent disclosure is not limited to these examples. Various values andmaterials used in these examples are illustrative. The description willbe given in the following order:

1. General description of an electrode material, an electrode, and abattery according to an embodiment of the present disclosure

2. Example 1 (an electrode material and an electrode according to anembodiment of the present disclosure)

3. Example 2 (variations of the electrode material and the electrodeprepared in Example 1)

4. Example 3 (a battery according to an embodiment of the presentdisclosure)

5. Example 4 (variations of the battery prepared in Example 3), etc.

[General Description of an Electrode Material, an Electrode, and aBattery According to an Embodiment of the Present Disclosure]

In an electrode material, an electrode, and a battery according to anembodiment of the present disclosure (hereafter, these may becollectively referred to as “an embodiment of the present disclosure”),a porous carbon material may be used for oxygen reduction at a pH of 3or more and 10 or less. Specifically, a porous carbon material may beused for oxygen reduction in an electrolyte having a pH of 3 or more and10 or less.

According to an embodiment of the present disclosure including thepreferred embodiment described above, the specific surface area of theporous carbon material may be 100 m²/g or more as measured by thenitrogen BET method and the pore volume of the porous carbon materialmay be 0.2 cm³/g or more as measured by the BJH method and 0.1 cm³/g ormore as measured by the MP method.

According to an embodiment of the present disclosure including thepreferred embodiments described above, the oxygen reduction startingpotential of the porous carbon material (i.e., electrode material,electrode, or positive electrode) may be more noble than 0.15 V asmeasured versus a Ag/AgCl reference electrode.

A battery according to an embodiment of the present disclosure includingthe preferred embodiments described above may include an electrolyticsolution having a pH of 3 or more and 10 or less. In order to maintainthe pH of 3 or more and 10 or less, for example, the electrolyticsolution may contain a buffer substance. Generally, any buffer substancemay be used as long as it has a pK_(a) of 4 or more and 10 or less. Thecontent of the buffer substance is preferably 0.2 mol or more per literof the electrolytic solution. The maximum content of the buffersubstance in the electrolytic solution may be, for example, the maximumsolubility of the buffer substance in the electrolytic solution. Morepreferably, the content of the buffer substance in the electrolyticsolution may be close to the maximum solubility of the buffer substancein the electrolytic solution.

In an electrode and a battery according to an embodiment of the presentdisclosure including the preferred embodiments described above, theporous carbon material may include an oxygen reduction catalystsupported on the porous carbon material. The oxygen reduction catalystmay be at least one material selected from the group consisting of noblemetals including platinum (Pt), transition-metal oxides, organometalliccomplexes and polymers thereof (specifically, e.g., a transition-metalporphyrin, phthalocyanine, a porphyrin polymer produced bypolymerization of transition-metal porphyrins, and a phthalocyaninepolymer produced by polymerization of phthalocyanines), perovskite, anda product of pyrolysis of a cobalt salt using polyacrylonitrile. Otherexamples of the oxygen reduction catalyst include LaBO₃ (B: Mn, Co)perovskite-type oxides, nitrides, and sulfides; and multi-componentperovskite oxides such as La_(1-x)A′_(x)Co_(1-y)Fe_(y)O₃ (where A′represents Sr or Ca, and x and y are 0.2 to 0.5). These oxygen reductioncatalysts are used for four-electron reduction of oxygen.

In an electrode and a battery according to an embodiment of the presentdisclosure including the preferred embodiments described above, theporous carbon material may be supported on a supporting member. Anelectrode according to an embodiment of the present disclosure includingthe preferred embodiments described above may be used as the positiveelectrode of a battery.

A specific method for measuring the oxygen reduction starting potentialof the porous carbon material (i.e., electrode material) is describedbelow. A test oxygen reduction electrode is prepared by forming a filmon a commercially-available grassy-carbon electrode (specifically,grassy-carbon electrode produced by BAS Inc.) by depositing a pasteprepared from a plant-derived porous carbon material. Then, the oxygenreduction ability of the test oxygen reduction electrode is evaluated byan electrochemical measurement while the test oxygen reduction electrodeis rotated at 1,000 rmp in an air-saturated phosphate buffer (pH of 7, 1mol/l) in order to supply the test oxygen reduction electrode withoxygen. The measurement apparatuses are a rotating disk electrode systemRDE-2 and an electrochemical analyzer ALS 701 produced by ALS Co., Ltd.The oxygen reduction starting potential of an electrode or a positiveelectrode prepared from the porous carbon material may be measured bylinear sweep voltammetry using a rotating disk electrode system RDE-2and an electrochemical analyzer ALS 701. Specifically, a potential atwhich the oxygen reduction current increases is measured.

According to an embodiment of the present disclosure, the porous carbonmaterial is made from a plant-derived material. Examples of theplant-derived material include, but are not limited to, chaff and strawof rice, barley, wheat, rye, Japanese barnyard millet, foxtail millet,and the like; coffee beans; tea leaves (e.g., green tea leaves and blacktea leaves); sugarcanes (specifically, bagasse); maize (specifically,corn cob); fruit peels (e.g., citrus fruit peels such as orange peels,grapefruit peels, and Japanese mandarin peels and banana peels); reeds;and wakame stems. In addition, vascular plants living on land,pteridophytes, bryophytes, algae, seaweeds, and the like may be used.These materials may be used, as the raw material of the porous carbonmaterial, alone or in a mixture of two or more types of these materials.The shape and form of the plant-derived material are not particularlylimited. For example, the plant-derived material such as chaff or strawmay be used without being processed or in the form of a dried product.Alternatively, the plant-derived material subjected to various processessuch as fermentation, roasting, and extraction in processing formanufacturing a beverage such as beer, Western liquor, and the like maybe used. In particular, straw and chaff that have been processed bythreshing or the like are preferably used in order to recycle industrialwaste. These straw and chaff that have been processed by threshing orthe like are easily available in large amounts from an agriculturalcooperative, an alcoholic beverage manufacturing company, a foodcompany, a food-processing company, or the like.

The silicon (Si) content of a porous carbon material prepared by beingtreated with an acid or an alkali after carbonization is less than 5% bymass, preferably 3% by mass or less, and more preferably 1% by mass orless. The silicon (Si) content of the raw material (i.e., aplant-derived material before carbonization) is preferably 5% by mass ormore.

The porous carbon material according to an embodiment of the presentdisclosure may be prepared by, for example, carbonizing a plant-derivedmaterial at 400° C. to 1,400° C. and subsequently treating thecarbonized material with an acid or an alkali. In a method for producingthe porous carbon material according to an embodiment of the presentdisclosure (hereafter, may be referred to as “method for producing theporous carbon material”), a material that is prepared by carbonizing aplant-derived material at 400° C. to 1,400° C. and that has not yet beentreated with an acid or an alkali is referred to as “precursor of aporous carbon material” or “carbonaceous substance”.

In the method for producing the porous carbon material, an activationtreatment may be performed after the acid or alkali treatment. Theactivation treatment may be performed before the acid or alkalitreatment. In the method for producing the porous carbon materialincluding the preferred method described above, depending on the type ofthe plant-derived material used, before being carbonized, theplant-derived material may be subjected to a heat treatment (preliminarycarbonization treatment) at a temperature (e.g., 400° C. to 700° C.)lower than the temperature for carbonization under exclusion of oxygen.This allows a tar component that is considered to be produced throughcarbonization to be extracted, which results in a reduction in theamount of tar component produced or removal of the tar componentproduced. A condition where oxygen is excluded may be achieved bypreparing an atmosphere of an inert gas such as nitrogen gas or argongas or a vacuum atmosphere or by “smothering” a plant-derived material.In the method for producing the porous carbon material, depending on thetype of the plant-derived material used, the plant-derived material maybe immersed in an alcohol (e.g., methyl alcohol, ethyl alcohol, orisopropyl alcohol) in order to reduce the contents of mineral componentsand moisture in the plant-derived material and to prevent an offensivesmell from being given off in the carbonization. In the method forproducing the porous carbon material, the preliminary carbonizationtreatment may be performed after the immersion treatment with analcohol. Examples of a material that is preferably heat-treated in aninert gas include plants that yield a large amount of pyroligneous acid(tar and light-oil components). Examples of a material that ispreferably subjected to the preliminary treatment with an alcoholinclude sea weeds, which contain iodine and various minerals inabundance.

In the method for producing the porous carbon material, a plant-derivedmaterial is carbonized at 400° C. to 1,400° C. The term “carbonization”generally refers to converting an organic substance (i.e., plant-derivedmaterial for the porous carbon material according to an embodiment ofthe present disclosure) into a carbonaceous substance by a heattreatment (e.g., see JIS M0104-1984). An example of an atmosphere usedfor carbonization is an atmosphere from which oxygen has been excluded.Specific examples thereof include a vacuum atmosphere, an atmosphere ofan inert gas such as nitrogen gas or argon gas, and an atmosphere inwhich a plant-derived material is “smothered”. The rate of temperaturerise until the carbonization temperature is achieved is, but not limitedto, 1° C./min or more, preferably 3° C./min or more, and more preferably5° C./min or more in the above-described atmosphere. The maximumcarbonization duration is, for example, but not limited to, 10 hours,preferably 7 hours, and more preferably 5 hours. The minimumcarbonization duration may be set to a time in which a plant-derivedmaterial is fully carbonized. A plant-derived material may bepulverized, if necessary, to a desired grain size or may be classified.The plant-derived material may be washed in advance. Alternatively,after a precursor of a porous carbon material or a porous carbonmaterial is prepared, it may be pulverized, if necessary, to a desiredgrain size or may be classified. In another case, after a porous carbonmaterial is subjected to the activation treatment, it may be pulverized,if necessary, to a desired grain size or may be classified. The finallyproduced porous carbon material may be subjected to a sterilizationtreatment. The type, components, and structure of a furnace used forcarbonization are not limited, and both a continuous-type furnace and abatch-type furnace may be used.

In the method for producing the porous carbon material, as describedabove, the number of micropores having a diameter smaller than 2 nm maybe increased by the activation treatment. Examples of the activationtreatment method include a gas-activation method and achemical-activation method. The gas-activation method herein refers to amethod in which a porous carbon material is heated for several tenminutes to several hours at 700° C. to 1,400° C., preferably 700° C. to1,000° C., and more preferably 800° C. to 1,000° C. in a gas atmosphereusing oxygen, water vapor, carbonic acid gas, air, or the like as anactivator to promote development of the microstructure of the volatilecomponents and carbon molecules in the porous carbon material. Morespecifically, the heating temperature may be selected appropriately onthe basis of the type of the plant-derived material used, the type andconcentration of the gas used, and the like. The chemical-activationmethod herein refers to a method in which a porous carbon material isactivated using zinc chloride, iron chloride, calcium phosphate, calciumhydroxide, magnesium carbonate, potassium carbonate, sulfuric acid, orthe like instead of oxygen, water vapor, or the like used in thegas-activation method, washed with hydrochloric acid, added in analkaline aqueous solution to adjust the pH of the porous carbonmaterial, and then dried.

In the method for producing the porous carbon material, siliconcomponents in the carbonized plant-derived material are removed by anacid or alkali treatment. Examples of the silicon components includesilicon oxides such as silicon dioxide, silicon oxide, and silicates. Aporous carbon material having a large specific surface area may beproduced by removing these silicon components in the carbonizedplant-derived material. In some cases, alternatively, the siliconcomponents in the carbonized plant-derived material can be removed by adry-etching method.

The porous carbon material according to an embodiment of the presentdisclosure includes a large number of pores, which are classified as“mesopores” having a diameter of 2 to 50 nm, “macropores” having adiameter of more than 50 nm, and “micropores” having a diameter of lessthan 2 nm. Specifically, the diameter of mesopores included in theporous carbon material in large numbers is, for example, 20 nm or lessand particularly 10 nm or less. The diameters of micropores included inthe porous carbon material in large numbers are, for example, about 1.9nm, about 1.5 nm, and about 0.8 to 1 nm. The pore volume of the porouscarbon material according to an embodiment of the present disclosure is0.2 cm³/g or more, preferably 0.3 cm³/g or more, and more preferably 0.5cm³/g or more as measured by the BJH method. The pore volume of theporous carbon material according to an embodiment of the presentdisclosure is 0.1 cm³/g or more, preferably 0.2 cm³/g or more, morepreferably 0.3 cm³/g or more, and further preferably 0.5 cm³/g or moreas measured by the MP method.

The specific surface area of the porous carbon material according to anembodiment of the present disclosure (hereafter, may be referred tosimply as “specific surface area”) is preferably 400 m²/g or more asmeasured by the nitrogen BET method in order to impart a betterfunctionality to the porous carbon material.

The nitrogen BET method is a method in which adsorbing molecules(herein, nitrogen molecules) are adsorbed to and desorbed from anabsorbent (herein, porous carbon material) to determine an adsorptionisotherm, and the adsorption isotherm is then analyzed by the BETequation represented by Equation (1). The specific surface area and porevolume may be calculated by this method. A specific method forcalculating the specific surface area by the nitrogen BET method isdescribed below. Nitrogen molecules serving as adsorbing molecules areadsorbed to and desorbed from a porous carbon material to determine anadsorption isotherm. Then, [p/{V_(a)(p₀−p)}] is calculated from theadsorption isotherm by Equation (1) or Equation (1′), which is adeformation of Equation (1), and plotted against the relativeequilibrium pressure (p/p₀). The resulting plotted line is considered tobe a straight line, and the slope s (=[(C−1)/(C×V_(m))]) and theintercept i (=[1/(C×V_(m))]) of the straight line are calculated by aleast-squares method. V_(m) and C are calculated from the slope s andthe intercept i by Equations (2-1) and (2-2), respectively. Then, thespecific surface area a_(sBET) is calculated from V_(m) by Equation (3)(see the manual for BELSORP-mini and BELSORP analysis software producedby BEL Japan, Inc., pp. 62-66). The nitrogen BET method is a measurementmethod conforming to JIS R 1626-1996 “measuring methods for the specificsurface area of fine ceramic powders by gas adsorption using the BETmethod”.V _(a)=(V _(m) ×C×p)/[(p ₀ −p){1+(C−1)(p/p ₀)}]  (1)[p/{V _(a)(p ₀ −p)}]=[(C−1)/(C×V _(m))](p/p ₀)+[1/(C×V _(m))]  (1′)V _(m)=1/(s+i)  (2-1)C=(s/i)+1  (2-2)a _(sBET)=(V _(m) ×L×σ)/22414  (3)

(where V_(a) represents the volume of nitrogen adsorbed; V_(m)represents the volume of nitrogen adsorbed on a monomolecular layer; prepresents the equilibrium pressure of nitrogen; p₀ represents thesaturated vapor pressure of nitrogen; L represents the Avogadro'snumber; and σ represents the adsorption cross section of nitrogen)

In order to calculate the pore volume V_(p) by the nitrogen BET method,for example, adsorption data of the adsorption isotherm are linearlyinterpolated, and an adsorption volume V at the relative pressure setfor calculating the pore volume is determined. The pore volume V_(p) canbe calculated from the adsorption volume V by Equation (4) (see themanual for BELSORP-mini and BELSORP analysis software produced by BELJapan, Inc., pp. 62-65). Hereafter, a pore volume determined by thenitrogen BET method may be simply referred to as “pore volume”.V _(p)=(V/22414)×(M _(g)/ρ_(g))  (4)

(where V represents the volume of nitrogen adsorbed at the relativepressure; M_(g) represents the molecular weight of nitrogen; and ρ_(g)represents the density of nitrogen)

The diameter of mesopores can be calculated as pore distribution fromthe rate of change in the pore volume relative to the pore diameter bythe BJH method or the like. The BJH method is a method that is widelyused as a method for analyzing pore distribution. The pore distributioncan be analyzed by the BJH method as follows. Nitrogen molecules servingas adsorbing molecules are adsorbed on and desorbed from a porous carbonmaterial to determine an adsorption isotherm. Then, the thickness of anadsorption layer created when the adsorbing molecules (e.g., nitrogenmolecules) are gradually adsorbed on and desorbed from pores filled withthe adsorption molecules and the inner diameter (twice as long as thecore radius) of pores created at that time are determined from theadsorption isotherm. The pore radius r_(p) is calculated by Equation(5), and the pore volume is calculated by Equation (6). The rate(dV_(p)/dr_(p)) of change of the pore volume is plotted against the porediameter (2r_(p)), which are calculated from the pore radius and thepore volume, to draw a pore distribution curve (see the manual forBELSORP-mini and BELSORP analysis software produced by BEL Japan, Inc.,pp. 85-88).r _(p) =t+r _(k)  (5)V _(pn) =R _(n) ×dV _(n) −R _(n) ×dt _(n) ×c×ΣA _(pj)  (6)where,R _(n) =r _(pn) ²/(r _(kn)−1+dt _(n))²  (7)

(where r_(p) represents the pore radius; r_(k) represents the coreradius ((inner diameter)/2) when an adsorption layer having a thicknesst is adsorbed on the inner wall of a pore having a radius r_(p) at thepressure; V_(pn) represents the pore volume at adsorption and desorptionof nitrogen in the n-th time; dV_(n) represents the amount of change involume at that time; dt_(n) represents the amount of change in thethickness t_(n) of the adsorption layer at adsorption and desorption ofnitrogen in the n-th time; r_(kn) represents the core radius at thattime; c is a fixed value; r_(pn) represents the pore radius atadsorption and desorption of nitrogen in the n-th time; and ΣA_(pj)represents an integrated value of the areas of the wall surfaces of thepores from j=1 to j=n−1)

The diameter of micropores may be calculated, for example, as a poredistribution from the rate of change in the pore volume relative to thepore diameter in accordance with the MP method. When pore distributionis analyzed by the MP method, nitrogen molecules are adsorbed on anddesorbed from a porous carbon material, and thereby an adsorptionisotherm is determined. The adsorption isotherm is converted (t-plot)into a curve showing pore volumes plotted against the thickness t of anadsorption layer. A pore distribution curve can be determined on thebasis of the curvature of the curve (amount of change in pore volumerelative to the amount of change in the thickness t of an adsorptionlayer) (see the manual for BELSORP-mini and BELSORP analysis softwareproduced by BEL Japan, Inc., pp. 72-73 and p. 82).

Specific examples of a method for treating the precursor of a porouscarbon material with an acid or an alkali include a method in which theprecursor of a porous carbon material is immersed in an aqueous solutionof an acid or an alkali and a method in which the precursor of a porouscarbon material is caused to react with an acid or an alkali in a gasphase. More specifically, examples of an acid used for the treatmentinclude acidic fluorine compounds such as hydrogen fluoride,hydrofluoric acid, ammonium fluoride, calcium fluoride, and sodiumfluoride. When a fluorine compound is used, the amount of fluorine ispreferably four times the amount of silicon contained in siliconcomponents included in the precursor of a porous carbon material. Theconcentration of an aqueous solution of the fluorine compound ispreferably 10% by mass or more. When hydrofluoric acid is used to removethe silicon components (e.g., silicon dioxide) included in the precursorof a porous carbon material, silicon dioxide reacts with hydrofluoricacid as shown by Chemical Formula (A) or (B) and then removed ashydrogen hexafluorosilicate (H₂SiF₆) or silicon tetrafluoride (SiF₄),and thus a porous carbon material is produced. Subsequently, the porouscarbon material may be washed and then dried.SiO₂+6HF→H₂SiF₆+2H₂O  (A)SiO₂+4HF→SiF₄+2H₂O  (B)

An example of an alkali used for the treatment is sodium hydroxide. Whenan aqueous solution of an alkali is used, the pH of the aqueous solutionmay be 11 or more. When an aqueous sodium hydroxide solution is used toremove the silicon components (e.g., silicon dioxide) included in theprecursor of a porous carbon material, silicon dioxide is caused toreact as shown by Chemical Formula (C) by heating the aqueous sodiumhydroxide solution and then removed as sodium silicate (Na₂SiO₃). Thus,a porous carbon material is produced. When the precursor is treated byreacting with sodium hydroxide in a gas phase, solid sodium hydroxide iscaused to react as shown by Chemical Formula (C) by being heated andthen removed as sodium silicate (Na₂SiO₃). Thus, a porous carbonmaterial is produced. Subsequently, the porous carbon material may bewashed and dried.SiO₂+2NaOH→Na₂SiO₂+H₂O  (C)

The porous carbon material according to an embodiment of the presentdisclosure may be, for example, a porous carbon material including poreshaving three-dimensional periodicity (porous carbon material having an“inverse-opal structure”) disclosed in Japanese Unexamined PatentApplication Publication No. 2010-106007. Specifically, the porous carbonmaterial includes three-dimensionally arranged spherical pores with anaverage diameter of 1×10⁻⁹ to 1×10⁻⁵ m and has a surface area of 3×10²m²/g or more. Preferably, the pores are arranged in the surface of theporous carbon material so as to correspond a crystalline structure or soas to correspond to the arrangement of atoms on the (111) plane of aface-centered cubic structure from a macroscopic point of view.

Examples of the supporting member on which the porous carbon material issupported include woven fabric and unwoven fabric made from variousnatural fiber or synthetic fiber; carbon/graphite-fiber-containing clothand carbon/graphite-fiber-based cloth; a sheet-like material producedfrom carbon fiber; and a foil-like material, a plate-like material, andmesh-like material that are composed of a metal or an alloy. Examples ofthe metal and the alloy include titanium, an titanium alloy, aluminium,an aluminium alloy, nickel, a nickel alloy, iron, and stainless steel.The porous carbon material may be supported on the supporting member by,for example, preparing a paste containing the porous carbon material,depositing the paste on one surface or both surfaces of the supportingmember by spraying, brush coating, printing, painting, spin coating, orthe like to form a porous-carbon-material layer composed of the paste,and then drying the porous-carbon-material layer. The amount of porouscarbon material supported on the supporting member may be, for example,0.05 to 5 mg per square centimeter of the surface of the supportingmember. The overall thickness of the supporting member including theporous carbon material supported thereon may be, for example, 10 μm to 1mm.

Examples of the battery according to an embodiment of the presentdisclosure or a battery in which the electrode material or the electrodeaccording to an embodiment of the present disclosure may be utilizedinclude the following:

(A) enzymatic biofuel cells that use organic matter such as sugar,alcohol, or cellulose as a fuel;

(B) microbial fuel cells used in wastewater treatment or sludgetreatment, in which a reaction is caused using organic matter containedin the wastewater or sludge to decompose the organic matter (Inmicrobial fuel cells, microorganisms are used as the negative electrodeand the electrode material according to an embodiment of the presentdisclosure may be used as the positive electrode. Energy produced bymicrobial decomposition of wastewater or sludge is extracted aselectrical energy);

(C) metal-air batteries that include a negative electrode with anegative-electrode-active material including a metal and an alloyedmaterial (Examples of the metal and the alloyed material used as thenegative-electrode-active material include alkali metals such aslithium, sodium, and potassium; Group 2 elements such as magnesium andcalcium; Group 13 elements such as aluminium; transition metals such aszinc and iron; and alloyed materials and compounds including any ofthese metals);

(D) various types of fuel cells such as

-   -   (i) a polymer electrolyte fuel cell (PEFC) that includes a fuel        electrode (negative electrode), a solid polymer membrane        (electrolyte), and an air electrode (positive electrode, which        may be the electrode according to an embodiment of the present        disclosure) that are merged together to form a        membrane-electrode assembly (MEA) interposed between conductive        plates called “bipolar plates” in which reaction-gas-feeding        passages are formed,    -   (ii) a phosphoric-acid fuel cell (PAFC) that includes a        separator impregnated with an electrolyte that is an aqueous        phosphoric acid (H₃PO₄) solution,    -   (iii) a molten carbonate fuel cell (MCFC) that uses carbonate        ions (CO₃ ²⁻) instead of hydrogen ions (H⁺) and includes a        separator impregnated with an electrolyte that is a molten        carbonate (e.g., lithium carbonate or potassium carbonate),    -   (iv) a solid-oxide fuel cell (SOFC) that includes, as an        electrolyte, stabilized zirconia having a high oxide-ion        permeability or an ion-conducting ceramic such as a perovskite        oxide of lanthanum or gallium and that allows oxide ions (O²⁻)        produced at the air electrode (positive electrode that is the        electrode according to an embodiment of the present disclosure)        to pass through the electrolyte and to react with hydrogen or        carbon monoxide at the fuel electrode to produce an electrical        energy,    -   (v) an alkaline fuel cell (AFC) that includes an ion-conductor        that is a hydroxide ion and a separator impregnated with an        alkaline electrolytic solution, the separator being interposed        between electrodes, and    -   (vi) a direct-methanol fuel cell (DMFC) such as a        direct-methanol fuel cell that includes a positive electrode        that is the electrode according to an embodiment of the present        disclosure and a fuel electrode (negative electrode) at which        methanol is directly oxidized;

as applications of the electrode material or the electrode according toan embodiment of the present disclosure,

(E) gas diffusion electrodes for oxygen reduction used in brineelectrolysis; and

(F) electrochemical sensors for detecting oxygen.

The above-described batteries, electrodes, and sensors may includeexisting components and have an existing structure.

The battery according to an embodiment of the present disclosure may beincorporated into an electronic device. Generally, the type of theelectronic device is not limited and may be portable or stationary.Specific examples of the electronic devices include a cellulartelephone, a mobile device, a robot, a personal computer, a gamemachine, a camera-integrated video tape recorder (VTR), avehicle-mounted device, various home-electric appliances, and industrialgoods.

EXAMPLES Example 1

Example 1 relates to an electrode material according to an embodiment ofthe present disclosure. The electrode material prepared in Example 1includes a plant-derived porous carbon material having an ability tocatalyze oxygen reduction.

In Example 1 and Examples 2 to 4 described below, the followingplant-derived porous carbon material was used. Specifically, aplant-derived material, that is, the raw material of the porous carbonmaterial, was rice chaff. The raw material, that is, chaff wascarbonized to be converted into a carbonaceous substance (precursor ofthe porous carbon material). Then, the carbonaceous substance wastreated with an acid to prepare the porous carbon material.

In order to prepare the porous carbon material, the plant-derivedmaterial was carbonized at 400° C. to 1,400° C. and the resultingmaterial was treated with an acid or alkali. Specifically, chaff wascarbonized (fired) in a nitrogen gas atmosphere at 800° C. to prepare aprecursor of the porous carbon material. Then, the precursor of theporous carbon material was treated with an alkali by being immersed inan aqueous sodium hydroxide solution (20 mass %) at 80° C. overnight toremove silicon components in the carbonized plant-derived material,washed with water and ethyl alcohol until the pH of 7 is achieved, andthen dried to prepare an intermediate of the porous carbon material. Theintermediate of the porous carbon material was heated to 900° C. in anitrogen gas atmosphere and thereby subjected to an activation treatmentwith water vapor. The resulting material was pulverized to a size of 4μm with a jet mill. Thus, the plant-derived porous carbon material ofExample 1 was prepared.

In Example 1, 0.1 g of the plant-derived porous carbon material, 250 μlof 10% Nafion (registered trademark), and 5 ml of 2-propanol were mixedto prepare a paste. Nafion, which served as a binder in Example 1, is aproduct of Sigma-Aldrich Japan K.K. Nafion is a perfluorocarbon materialincluding a hydrophobic Teflon skeleton composed of carbon-fluorinebonds and a perfluoro side chain having a sulfonic group, that is, acopolymer of tetrafluoroethylene and aperfluoro[2-(fluorosulfonylethoxy)propylvinyl ether]. The paste wasdeposited on the commercially-available glassy carbon electrodedescribed above to form a film. Thus, a test oxygen reduction electrodewas prepared. Then, the oxygen reduction ability of the test oxygenreduction electrode was evaluated by an electrochemical measurementwhile the test oxygen reduction electrode was rotated in anair-saturated phosphate buffer (pH of 7, 1 mol/l) in order to supply thetest oxygen reduction electrode with oxygen. The measurement apparatusused were a rotating disk electrode system RDE-2 and an electrochemicalanalyzer ALS 701 produced by ALS Co., Ltd.

Test oxygen reduction electrodes for Comparative Examples were preparedusing the following materials instead of the porous carbon material andevaluated on their oxygen reduction abilities. In addition, as areference example, the oxygen reduction ability of a platinum electrodewas evaluated.

Comparative Example 1A: Vapor-Grown Carbon Fiber “VGCF-H”® Produced byShowa Denko K.K.

Comparative Example 1B: Nitrogen-Doped Carbon Nanotube (NitrogenContent: 2.5 Mass %) Produced by NANO-MIR CO. LTD.

Comparative Example 1C: High-Surface-Area Graphitized Mesoporous CarbonProduced by Sigma-Aldrich Japan K.K. (Product No. 699624)

Comparative Example 1D: Mesoporous Carbon Produced by Sigma-AldrichJapan K.K. (Product No. 402110)

FIGS. 1 and 2 show the measurement results. In FIGS. 1 and 2, thehorizontal axis represents voltage (units: V) and the vertical axisrepresents current (units: 10⁻⁴ A in FIG. 1 and 10⁻⁵ A in FIG. 2). InFIG. 1, the line numbered “1” shows the data of Example 1, the linenumbered “2” shows the data of Comparative Example 1A, the line numbered“3” shows the data of Comparative Example 1B, the line numbered “4”shows the data of Comparative Example 1C, and the line numbered “5”shows the data of Comparative Example 1D. FIG. 2 shows the result ofevaluation of the oxygen reduction ability of the platinum electrode asa reference example. The comparison of the results shown in FIGS. 1 and2 shows that the electrode material of Example 1 which included theplant-derived porous carbon material had a catalytic ability (ability tocatalyze oxygen reduction) comparable to that of the platinum electrodeunder a pH-neutral condition (in Example 1, pH of 7) because the oxygenreduction starting potentials of the electrode material of Example 1 wassubstantially equal to that of the platinum electrode. In other words,the electrode material of Example 1 which included the plant-derivedporous carbon material had an overvoltage substantially equal to that ofthe platinum electrode under a pH-neutral condition. Thus, in theelectrode material of Example 1 which included the plant-derived porouscarbon material, that is, in an electrode prepared using the electrodematerial or in an electrode of a battery including such an electrode,the oxygen reduction starting potential of the porous carbon material(i.e., electrode material, electrode, or positive electrode) was morenoble than 0.15 V as measured versus a Ag/AgCl reference electrode. FIG.1 shows that the electrode material of Example 1 which included theplant-derived porous carbon material produced a larger current outputthan the electrode materials of Comparative Example 1A, ComparativeExample 1C, and Comparative Example 1D and had a higher oxygen reductionstarting potential than the electrode materials of Comparative Example1A, Comparative Example 1B, Comparative Example 1C, and ComparativeExample 1D. Thus, it is confirmed that the electrode material of Example1 had a markedly higher performance than the electrode materials ofComparative Examples.

Table 1 shows the specific surface areas of the plant-derived porouscarbon material of Example 1 and the material used in ComparativeExample 1A as measured by the nitrogen BET method (“specific surfacearea” in Table 1, units: m²/g), the pore volume of these materials asmeasured by the nitrogen BET method (“volume by BET method” in Table 1,units: cm³/g), the pore volume as measured by the MP method (“MP method”in Table 1, units: cm³/g), and the pore volume as measured by the BJHmethod (“BJH method” in Table 1, units: cm³/g). FIGS. 3A and 3B show themeasurement results of the cumulative pore volume. In FIGS. 3A and 3B,the plotted curve denoted by “A” shows the data of Example 1 and theplotted curve denoted by “B” shows the data of Example 2. Themeasurement result of a sample having a significantly small specificsurface area, such as the sample of Comparative Example 1A, shows thebehavior shown by “B” in FIG. 3A. In reality, negative values wereobserved and therefore the plotted line is invisible.

TABLE 1 Specific surface Volume by area BET method MP method BJH methodExample 1 1220 0.998 0.456 0.642 Comparative 13 0.081 0.0 0.087 Example1A

The results shown in Table 1 show that the plant-derived porous carbonmaterial of Example 1 had a pore structure quite different from that ofthe material used in Comparative Example 1A. Specifically, theplant-derived porous carbon material of Example 1 had a specific surfacearea of 100 m²/g or more as measured by the nitrogen BET method, a porevolume of 0.2 cm³/g or more as measured by the BJH method, and a porevolume of 0.1 cm³/g or more as measured by the MP method. Thus, theelectrode material of Example 1 sufficiently exhibited an oxygenreduction ability under a pH-neutral condition and produced a largecurrent output presumably because of this unique pore structure of theplant-derived porous carbon material.

As described above, an electrode material having a remarkable ability tocatalyze oxygen reduction may be produced by preparing the electrodematerial using the plant-derived porous carbon material of Example 1. Anelectrode prepared using the plant-derived porous carbon material ofExample 1 may have an overvoltage for oxygen reduction substantiallyequal to that of a platinum electrode and produce a large currentoutput. This electrode may sufficiently exhibit an oxygen reductionability under a pH-neutral condition, which increases the versatility ofthe electrode. Thus, this electrode may have a high applicability tovarious types of devices and apparatuses in which an oxygen reductionelectrode capable of exhibiting an oxygen reduction ability under apH-neutral condition is anticipated.

Example 2

In Example 2, a variation of the electrode material of Example 1 isdescribed. In an electrode material of Example 2, a porous carbonmaterial includes an oxygen reduction catalyst supported thereon.Specifically, iron phthalocyanine (FePc) andcobalt(II)tetra(methoxyphenyl)porphyrin (CoTMPP) were used as oxygenreduction catalysts.

In Example 2, 0.1 g of the plant-derived porous carbon material preparedin Example 1, 250 μl of 10% Nafion, 5 ml of 2-propanol, and 0.05 g ofiron phthalocyanine (FePc) or cobalt(II)tetra(methoxyphenyl)porphyrin(CoTMPP) were mixed to prepare a paste. Then, a test oxygen reductionelectrode was prepared using the paste as in Example 1. The oxygenreduction ability of the test oxygen reduction electrode was evaluatedas in Example 1.

FIG. 4 shows the evaluation results of a test oxygen reduction electrodeprepared using iron phthalocyanine (FePc), which are shown by the curves“A”. FIG. 5 shows the evaluation results of a test oxygen reductionelectrode prepared using cobalt(II)tetra(methoxyphenyl)porphyrin(CoTMPP), which are shown by the curves “B”. When the plant-derivedporous carbon material prepared in Example 1 was used in combinationboth with FePc and with CoTMPP, the test oxygen reduction electrode hada higher performance than that prepared in Example 1 in which theplant-derived porous carbon material was used alone (shown by the curves“C” in FIGS. 4 and 5).

Example 3

Example 3 relates to an electrode according to an embodiment of thepresent disclosure and to a battery (specifically, metal-air battery)according to an embodiment of the present disclosure. An electrodeprepared in Example 3 included a plant-derived porous carbon materialhaving an ability to catalyze oxygen reduction. A battery prepared inExample 3 included a positive electrode (cathode) including aplant-derived porous carbon material having an ability to catalyzeoxygen reduction. Specifically, the battery of Example 3 was analuminium-air primary battery that included a negative electrode (anode)including a material containing aluminium and that generated electricpower using oxygen in the air as a positive-electrode-active material.As described above, the plant-derived porous carbon material used wasthe plant-derived porous carbon material prepared in Example 1. Theporous carbon material was supported on a supporting member(specifically, in Example 3, sheet-like material composed of carbonfiber). The electrode of Example 3 was used as the positive electrode ofthe battery.

In the related art, there have been studies on a method for promoting acathodic reaction using an alkaline solution as an electrolytic solutionin order to increase the output of an aluminium-air battery. However,the positive electrode of the aluminium-air battery using an alkalinesolution as an electrolytic solution may become degraded because severecorrosion of aluminium occurs under an alkaline condition and becausethe alkaline electrolytic solution absorbs carbon dioxide in the air andconsequently becomes neutralized gradually.

Thus, these issues may be addressed using the electrode (positiveelectrode) of Example 3 that causes oxygen to be reduced under apH-neutral condition.

Specifically, the aluminium-air battery of Example 3 included anelectrolytic solution having a pH of 3 or more and 10 or less. In orderto maintain the electrolytic solution at a pH of 3 or more and 10 orless, the electrolytic solution may contain a buffer substance or thelike. Generally, any buffer substance may be used as long as it has apK_(a) of 4 or more and 10 or less.

An aluminium-air battery includes a separator interposed between anegative electrode and a positive electrode. The negative electrode, thepositive electrode, the separator interposed between the negativeelectrode and the positive electrode, and the like are immersed in anelectrolytic solution. In other words, the separator constitutes anelectrolyte layer that is filled with the electrolytic solution and thatallows aluminium ions to migrate between the negative electrode and thepositive electrode. Examples of the material of the separator include aporous membrane composed of polyethylene oxide, polyacrylic acid,poly(vinyl alcohol), polyethylene, or polypropylene, various nonwovenfabrics, paper, and cellulose. Examples of the material of the nonwovenfabrics include, but are not limited to, various high-molecular-weightorganic compounds such as polyolefin, polyester, cellulose, andpolyacrylamide.

The material containing aluminium that constitutes the negativeelectrode may be a material containing aluminium as a main component.Specific examples of such a material include aluminium and variousaluminium alloys. The shape of the negative electrode may beappropriately selected if necessary and may be, for example, foil-like,sheet-like, or plate-like. More specifically, the negative electrodeused in Example 3 was an aluminium foil. The negative electrode may bearranged to be replaceable if necessary. The aluminium-air batterypreferably has a structure that allows insoluble matter produced as aby-product to be removed when the negative electrode is replaced.

The positive electrode and negative electrode are each connected to acurrent collector. The current collectors are typically composed of ametal mesh. The material of the metal mesh is not particularly limitedand any material may be used as long as it is capable of withstandingthe operating conditions for the aluminium-air battery. For example,titanium (Ti), nickel (Ni), and stainless steel (e.g., SUS304) may beused. The pore size of the metal mesh and the like are also notparticularly limited and may be set appropriately. The currentcollectors have an electrolytic-solution permeability.

The electrolytic solution preferably has a pH of 3 or more and 10 orless and typically includes a buffer substance having a pK_(a) of 4 ormore and 10 or less. Examples of the buffer substance include citricacid, ammonium chloride, phosphoric acid,tris(hydroxymethyl)aminomethane, a compound having an imidazole ring,dihydrogen phosphate ions (H₂PO₄ ⁻),2-amino-2-hydroxymethyl-1,3-propanediol (abbreviated as “tris”),2-(N-morpholino)ethanesulfonic acid (MES), cacodylic acid, carbonic acid(H₂CO₃), hydrogen citrate ion, N-(2-acetamide)imino diacetate (ADA),piperazine-N,N′-bis(2-ethanesulfonic acid) (PIPES),N-(2-acetamide)-2-aminoethanesulfonic acid (ACES),3-(N-morpholino)propanesulfonic acid (MOPS),N-2-hydroxyethylpiperazine-N′-2-ethanesulfonic acid (HEPES),N-2-hydroxyethylpiperazine-N′-3-propanesulfonic acid (HEPPS),N-[tris(hydroxymethyl)methyl]glycine (abbreviated as “tricine),glycylglycine, and N,N-bis(2-hydroxyethyl)glycine (abbreviated as“bicine”). Examples of a compound from which dihydrogen phosphate ions(H₂PO₄ ⁻) are produced include sodium dihydrogenphosphate (NaH₂PO₄) andpotassium dihydrogenphosphate (KH₂PO₄). Examples of the compound havingan imidazole ring include imidazole, triazole, a pyridine derivative, abipyridine derivative, and an imidazole derivative (e.g., histidine,1-methylimidazole, 2-methylimidazole, 4-methylimidazole,2-ethylimidazole, imidazole-2-carboxylic acid ethyl ester,imidazole-2-carboxaldehyde, imidazole-4-carboxylic acid,imidazole-4,5-dicarboxylic acid, imidazole-1-yl-acetic acid,2-acetylbenzimidazole, 1-acetylimidazole, N-acetylimidazole,2-aminobenzimidazole, N-(3-aminopropyl)imidazole,5-amino-2-(trifluoromethyl)benzimidazole, 4-azabenzimidazole,4-aza-2-mercaptobenzimidazole, benzimidazole, 1-benzylimidazole, or1-butylimidazole). If necessary, the electrolytic solution may include aneutralizer, which is at least one acid selected from the groupconsisting of hydrochloric acid (HCl), acetic acid (CH₃COOH), phosphoricacid (H₃PO₄), sulfuric acid (H₂SO₄), and the like. The electrolyticsolution may be composed of a substance containing halide ions (e.g.,chloride ions, bromide ions, iodide ions, or fluoride ions) or the like.For example, when an electrolytic solution is composed of a substancecontaining chloride ions, the electrolytic solution is composed of NaCl,KCl, or the like. The electrolytic solution may include ionic liquid.Any ionic liquid known in the related art may be used if necessary.

A gas-liquid separation membrane constituting a container that housesthe electrolytic solution may be, but is not limited to, apolytetrafluoroethylene (PTFE) membrane or the like. The shape of thecontainer may be selected appropriately if necessary. Examples of theshape of a battery case (container) that houses a positive electrode, anegative electrode, a separator, an electrolytic solution, and the likeinclude a coin shape, a plate shape, a cylinder shape, and amultilayer-body shape. The battery case may have an open-to-the-airstructure that allows the positive electrode and the like to be fullyexposed to the air or a confined structure that includes a gas(air)-introduction tube and an exhaustion tube.

When an aluminium-air battery is producing electric power, the reactionsshown by Formulae (31) to (33) occur at its negative electrode.Al→Al³⁺+3e ⁻  (31)Al³⁺+6H₂O→[Al(H₂O)₆]³⁺  (32)[Al(H₂O)₆]³⁺→[Al(OH)₆]³⁻+6H⁺  (33)

Equations (32) and (33) lead to the following equation.Al³⁺+6H₂O→[Al(OH)₆]³⁻+6H⁺  (34)

Thus, Al³⁺ migrates from the negative electrode to the positiveelectrode through the separator, which generates electric energy. In thepositive electrode, H⁺ transported through the separator filled with theelectrolytic solution and electrons transported from the negativeelectrode cause oxygen in the air to be reduced. Thus, water isproduced.

As shown in Equation (34), protons accumulate on the surface of thenegative electrode. If no measures are taken to avoid the accumulation,the pH of the surface of the negative electrode is reduced and, as aresult, self-corrosion of aluminium may occur, which disadvantageouslypromotes generation of hydrogen gas. When the electrolytic solutioncontains a buffer substance having pK_(a) of 4 or more and 10 or less,the action of the buffer substance allows the pH of the surface of thenegative electrode to be maintained substantially neutral (e.g., pH of 3or more and 10 or less). In this case, self-corrosion of aluminium andthe resulting promotion of the generation of hydrogen gas do not occur.FIG. 6 shows the relationship between the corrosion rate of aluminiumand pH. At a pH of 3 or more and 10 or less, little corrosion occurs orthe corrosion rate is extremely small, and therefore the generation ofhydrogen gas may be suppressed.

In Example 3, an electrode was prepared by the following method.Specifically, 1.0 g of the plant-derived porous carbon material preparedin Example 1, 0.5 g of a vapor-grown carbon fiber “VGCF-H”, 0.25 g ofiron phthalocyanine, 0.1 g of poly(vinylidene fluoride) (PVDF), and 8 mlof an N-methyl-2-pyrrolidone (NMP) solvent were mixed and then kneadedto prepare a paste. The vapor-grown carbon fiber “VGCF-H” served as aconduction-assisting agent, iron phthalocyanine served as an oxygenreduction catalyst, and PVDF served as a binder. The paste was appliedto a supporting member, and the resulting supporting member was dried toprepare an electrode of Example 3. The vapor-grown carbon fiber “VGCF-H”was used in order to improve electric conductivity and to increase easeof film-formation.

The oxygen reduction ability of the electrode of Example 3 was evaluatedusing the same apparatus as in Example 1 by linear sweep voltammetry,which is a method in which a current at an electrode is measured whilethe potential of the electrode is continuously changed. The electrode ofExample 3 was subjected to an electrochemical measurement in anair-saturated aqueous NaCl solution (4 mol/l). FIG. 7 shows the results.It was confirmed that the electrode of Example 3 had a good performanceas an electrode. Specifically, the electrode of Example 3 had anelectric current density of about 0.2 A/cm², which was higher than thehighest electric current density (40 mA/cm²) that has been reported todate.

Then, a metal-air battery including a negative electrode with anegative-electrode-active material including a metal and an alloyedmaterial was prepared. Specifically, an aluminium-air primary batteryincluding a negative electrode (anode) composed of aluminium (see theschematic cross-sectional view thereof shown in FIG. 8A) was preparedand then subjected to an electrochemical evaluation. More specifically,the battery of Example 3 included a positive electrode (cathode) 11 thatwas the electrode according to an embodiment of the present disclosure,a negative electrode (anode) 12 composed of aluminium, a separator 13composed of nonwoven fabric, which was interposed between the positiveelectrode 11 and the negative electrode 12, an electrolytic solution 14,a battery case 10 that housed the above-described components, andcurrent collectors 11′ and 12′ composed of a titanium mesh, which wereattached to the positive electrode 11 and the negative electrode 12,respectively. The electrolytic solution 14 was an aqueous sodiumchloride solution (3 mol/l). The battery case 10 composed of a PTFE filmalso served as a container constituted by a gas-liquid separationmembrane. The negative electrode 12 was a square aluminium foil having asize of 10 mm×10 mm×0.17 mm (thickness). The positive electrode 11 wasthe electrode of Example 3 having a size of 10 mm×10 mm×0.2 mm(thickness). The cell voltage of the battery was recorded while thebattery was controlled so that a predetermined load was applied to thebattery and a certain amount of current (2 mA) flowed. FIG. 9 shows theresults.

For comparison, 1.0 g of vapor-grown carbon fiber “VGCF-H”, which wasthe material used in Comparative Example 1A, 0.1 g of poly(vinylidenefluoride) (PVDF), and 8 ml of N-methyl-2-pyrrolidone (NMP) were mixedand then kneaded to prepare a paste. The paste was applied to asupporting member, and then the resulting supporting member was dried toprepare an electrode of Comparative Example 3. Subsequently, analuminium-air battery was prepared using this electrode and thensubjected to an electrochemical evaluation as in Example 3. FIG. 9 showsboth the results of electrochemical evaluations in Example 3 andComparative Example 3. In FIG. 9, the line denoted by “A” shows the dataof Example 3 and the line denoted by “B” shows the data of ComparativeExample 3.

FIG. 9 shows that the aluminium-air battery of Comparative Example 3 hadlittle performance as a battery and, on the other hand, thealuminium-air battery of Example 3 had a good performance. The outputcharacteristics of the aluminium-air battery of Example 3 weredetermined at a battery voltage of 0.95 V. As a result, it was confirmedthat the aluminium-air battery of Example 3 continued producing electricpower for 3 hours or more since it started generating electric power(started electric discharge). The maximum electric current densitymeasured at a battery voltage of 0.7 V was about 0.070 A/cm² and theoutput power was about 50 mW/cm².

As shown by the schematic cross-sectional view in FIG. 8B, an edge ofthe separator 13 may protrude from the battery case 10. In this case, aspace containing the positive electrode and a space containing thenegative electrode can be completely separated. This avoids the reactionproduct (aluminium hydroxide) produced at the negative electrode frommoving to the positive-electrode side, which elongates the service lifeof the battery.

The negative electrode 12 of the aluminium-air battery may be arrangedto be replaceable. As shown in FIG. 10, the negative electrode 12 ishoused in a sack-shaped membrane 15, the negative electrode 12 housed inthe membrane 15 is housed in a cartridge 16, and the cartridge 16 ishoused in a cartridge-housing portion 18. The sack-shaped membrane 15allows the electrolytic solution 14 to pass therethrough. Thecartridge-housing portion 18 is disposed on the separator 13. Thereference numerals 17A and 17B denote extrusion units. Thecartridge-housing portion 18 includes a cartridge-insertion port 18Athrough which the cartridge 16 is inserted from the outside to theinside and a cartridge-ejection port 18B through which the cartridge 16is ejected outward.

FIG. 11A shows the cartridge 16 that has not yet been used. FIG. 11Bshows the cartridge 16 in which the negative electrode 12 serving as a“fuel” is depleted. FIG. 11C shows the aluminium-air battery in whichthe negative electrode 12 in the cartridge 16 is depleted. After thenegative electrode 12 is depleted, the sack-shaped membrane 15 containsa by-product that is aluminium hydroxide 19 confined therein. In FIGS.11B, 11C, and 12, the reference numeral 17C denotes a spring forextrusion which is fixed to the extrusion units 17A and 17B at therespective ends of the spring. In FIG. 11A, the spring 17C is omitted.The extrusion unit 17A is fixed to the cartridge 16. The extrusion unit17B, which is pressed by the spring 17C, presses the negative electrode12 against the separator 13.

A used cartridge 16 may be replaced with an unused cartridge 16 asdescribed below. As shown in FIG. 12, the cartridge-insertion port 18Ais opened, and the unused cartridge 16 is inserted into thecartridge-housing portion 18 through the cartridge-insertion port 18A.The used cartridge 16 is extruded outward through the cartridge-ejectionport 18B. When the used cartridge 16 is completely extruded through thecartridge-ejection port 18B, the unused cartridge 16 is set at apredetermined position as shown in FIG. 10. In this unused cartridge 16,the extrusion unit 17B presses the negative electrode 12 against theseparator 13.

Alternatively, the negative electrode may be composed of an aluminiumalloy such as Al—Li, Al—Mg, Al—Sn, or Al—Zn. In another case, zinc, azinc alloy, magnesium, and a magnesium alloy may also be used. Analuminium-air battery having a good performance comparable to that ofthe aluminium-air battery of Example 3 was prepared using anelectrolytic solution prepared by adding, as a buffer substance, 1.0 molof imidazole per liter of the electrolytic solution. An aluminium-airbattery having a good performance comparable to that of thealuminium-air battery of Example 3 was prepared using an electrolyticsolution prepared by adding, as a buffer substance, 1.0 mol of citricacid per liter of the electrolytic solution.

Example 4

In Example 4, variations of the electrode and the battery of Example 3are described. A battery prepared in Example 4 also included a positiveelectrode (cathode) including the plant-derived porous carbon materialhaving an ability to catalyze oxygen reaction. The positive electrodeincluded the same components and had the same structure as the electrodedescribed in Example 3. The battery of Example 4 was an enzymaticbiofuel cell shown by the schematic cross-sectional view thereof in FIG.13.

The enzymatic biofuel cell of Example 4 was an immersion-type fuel cellincluding a positive electrode (air electrode, cathode) 21 that was theelectrode according to an embodiment of the present disclosure and anegative electrode (fuel electrode, anode) 22, which were both being incontact with an electrolyte. An oxygen-reduction enzyme was present onthe surface of the negative electrode 22. The expression “the surface ofan electrode” herein collectively refers to the outer surface of theelectrode and the inner surfaces of pores that are present inside theelectrode. Current collectors 21′ and 22′ were arranged to be in contactwith the positive electrode 21 and the negative electrode 22,respectively. The current collectors 21′ and 22′ had, for example, amesh-like shape and thus allowed an electrolytic solution and air topass therethrough. As shown in FIG. 13, for example, the inner surfaceof the positive electrode 21 was arranged to be in contact with a liquidphase (solution) and the outer surface of the positive electrode 21 wasarranged to be in contact with a gas phase (air) via the currentcollector 21′ interposed between the positive electrode 21 and the gasphase (air). The inner surface of the negative electrode 22 was arrangedto be in contact with a liquid phase (solution) and the outer surface ofthe negative electrode 22 was arranged to be in contact with a liquidphase (solution) via the current collector 22′ interposed between thenegative electrode 22 and the liquid phase. A cathode-solution section24 and an anode-solution section 25 were provided in the peripheries ofthe positive electrode 21 and the negative electrode 22, respectively. Aseparator 23 composed of unwoven fabric was interposed between thecathode-solution section 24 and the anode-solution section 25. Thebattery further included a fuel-solution-introduction port 27 throughwhich a fuel solution 26 was introduced. The fuel-solution-introductionport 27 was communicated with the anode-solution section 25.

The negative electrode 22 was a fuel electrode, which was prepared byfixing an oxidoreductase on the surface of an electrode composed of aconductive porous material or the like. Any material known in therelated art may be used as the conductive porous material constitutingthe negative electrode 22. In particular, carbon-based materials such asporous carbon, carbon pellets, carbon felt, carbon paper, carbon fiber,and layered carbon particles may be preferably used. The porous carbonmay be the plant-derived porous carbon material according to anembodiment of the present disclosure. When the fuel component isglucose, the enzyme fixed to the surface of the negative electrode 22may be glucose dehydrogenase (GDH) that allows glucose to be decomposed.When the fuel component is a monosaccharide such as glucose, thenegative electrode preferably further includes a coenzyme oxidase or anelectron-transfer mediator fixed to the surface thereof together with anoxidase such as GDH which promotes oxidation of a monosaccharide andwhich allows the monosaccharide to be decomposed. The coenzyme oxidaseoxidizes a coenzyme reduced by an oxidase (e.g., NAD⁺ or NADP⁺) and areduced form of a coenzyme (e.g., NADH or NADPH). An example of acoenzyme oxidase is diaphorase. When a coenzyme is again converted intoits oxidized form, electrons are produced due to the action of thecoenzyme oxidase. The electrons are transferred from the coenzymeoxidase to the negative electrode 22 via the electron-transfer mediator.

A compound including a quinone skeleton is preferably used as theelectron-transfer mediator. More preferably, a compound including anaphthoquinone skeleton is used. Specifically,2-amino-1,4-naphthoquinone (ANQ), 2-amino-3-methyl-1,4-naphthoquinone(AMNQ), 2-methyl-1,4-naphthoquinone (VK3),2-amino-3-carboxy-1,4-naphthoquinone (ACNQ), and the like may be used.Examples of the compound including a quinone skeleton include thecompound including a naphthoquinone skeleton, anthraquinone, and itsderivatives. If necessary, one or more other compounds that serve as theelectron-transfer mediator may be fixed on the surface of the negativeelectrode 22 together with the compound including a quinone skeleton.

When the fuel component is a polysaccharide, a degrading enzyme thatpromotes decomposition (e.g., hydrolysis) of the polysaccharide toproduce a monosaccharide such as glucose is desirably fixed on thesurface of the negative electrode 22 together with the above-describedoxidase, coenzyme oxidase, coenzyme, and electron-transfer mediator.Herein, the term “polysaccharide” is used in the broad sense of the termand refers to any carbohydrate that yields two or more molecules ofmonosaccharides by hydrolysis. Examples of the “polysaccharide” includeoligosaccharides such as a disaccharide, a trisaccharide, and atetrasaccharide. Specific examples thereof include starch, amylose,amylopectin, glycogen, cellulose, maltose, sucrose, and lactose. Thesepolysaccharides are composed of two or more monosaccharides linked toone another. Each of these polysaccharides includes glucose as amonosaccharide as a linking unit.

Starch contains amylose and amylopectin. Thus, starch is a mixture ofamylose and amylopectin. For example, when glucoamylase is used as apolysaccharide-degrading enzyme and glucosedehydrogenase is used as amonosaccharide-degrading oxidase, the fuel component may be apolysaccharide that can be decomposed into glucose by glucoamylase.Examples of such a polysaccharide include starch, amylose, amylopectin,glycogen, and maltose. Glucoamylase is a degrading enzyme that causeshydrolysis of an α-glucan such as starch to produce glucose.Glucosedehydrogenase is an oxidase that causes β-D-glucose to beoxidized into D-glucono-δ-lacton.

The negative electrode 22 is not limited to an electrode including anoxidoreductase fixed on the surface thereof. As long as anoxidoreductase is present on the electrode surface, for example, anelectrode including an oxidoreductase and microorganisms, which serve asreaction catalysts, attached on the electrode may also be used.

The fuel solution 26 is a solution containing a fuel component such as asugar, an alcohol, an aldehyde, a lipid, or a protein or is a solutioncontaining at least one of these fuel components. Specific examples ofthe fuel component include sugars such as glucose, fructose, andsorbose; alcohols such as methanol, ethanol, propanol, glycerin, andpoly(vinyl alcohol); aldehydes such as formaldehyde and acetaldehyde;and organic acids such as acetic acid, formic acid, and pyruvic acid.Fats, proteins, and the above-described organic acids that areintermediate products of sugar metabolism, and the like may also be usedas the fuel component. The fuel solution 26 may further include anelectrolyte that serves as a proton conductor.

More specifically, in Example 4, the fuel solution 26 was a phosphatebuffer (1 mol/l) containing 0.8 mol/l of glucose. Then,glucosedehydrogenase (GDH) was fixed to the surface of the negativeelectrode 22 including a conductive porous material. The cell voltage ofthe battery of Example 4 was recorded as in Example 3. That is, the cellvoltage of the battery of Example 4 was recorded while the battery wascontrolled so that a predetermined load was applied to the battery and acertain amount of current flowed. FIG. 14 shows the results. It wasconfirmed that the enzymatic biofuel cell of Example 4 had a goodperformance.

The battery of Example 4 may be applied to a battery having a“single-cell” structure in which the battery includes a single cell orto a battery in which a plurality of cells are connected to one anotherin series or in parallel. When a battery includes a plurality of cells,the cells may share a single fuel-solution-introduction port. As shownin FIG. 15, the positive electrode 21 and the negative electrode 22 maybe arranged to be in contact with the separator 23. In this case, theseparator is impregnated with the solution, and thus the inner surfacesof the positive electrode 21 and the negative electrode 22 are arrangedto be in contact with the solution.

A permselective membrane may be used instead of the separator composedof nonwoven fabric. The permselective membrane has a water permeabilityand is capable of suppressing the permeation of the fuel component andthe like contained in the fuel solution 26. The fuel solution 26introduced into the anode-solution section 25 is then introduced intothe cathode-solution section 24 through the permselective membrane. Thepermselective membrane may also suppress permeation of components otherthan the fuel component contained in the fuel solution 26. Inparticular, the permselective membrane preferably suppress permeation ofan enzyme or a mediator eluted into the fuel solution 26. Thissuppresses transfer of the enzyme and the mediator that are present ateach electrode to another electrode side, which results in suppressionof degradation of battery characteristics. The permselective membranemay suppress permeation of a substance that has an inhibitory effect onthe positive electrode 21. For example, when the fuel solution 26 is acommercially-available beverage, the permselective membrane may alsosuppress permeation of a calorie-free sweetener and permeation of sugars(e.g., fructose) that are difficult to be oxidized by the enzyme of thenegative electrode 22. This suppresses degradation of batterycharacteristics and thus increases power-generation efficiency.

Examples of the permselective membrane include a cellulose membrane anda synthetic polymer membrane. Specific examples of the cellulosemembrane include regenerated cellulose membranes (RC) such ascuprammonium rayon (CR) and saponified cellulose acetate (SCA);surface-modified regenerated cellulose membranes such as a hemophanmembrane and a vitamin E-coated membrane; and cellulose acetate (CA)membranes such as cellulose diacetate (CDA) and cellulose triacetate(CTA). Examples of the synthetic polymer membrane includepolyacrylonitrile (PAN), poly(methyl methacrylate) (PMMA),ethylene-vinylalcohol copolymer (EVA), a polysulfone (PS), a polyamide(PA), and a polyester-polymer alloy.

The average pore size of the permselective membrane may be, for example,0.5 μm or less in order to efficiently suppress permeation of the fuelcomponent. The average pore size of the permselective membrane ispreferably 100 nm or less, more preferably 20 nm or less, and furtherpreferably 10 nm or less in order to enhance the effect of suppressingpermeation of the fuel component and to also suppress permeation ofcomponents other than the fuel component, such as an enzyme and amediator. Desirably, the permselective membrane is provided so that theionic conductivity between the negative electrode 22 and the positiveelectrode 21 is set to 0.1 S/cm or more, that is, so that the internalresistance of the battery is set to 10Ω or less. This reduces the lossof power generated. The ionic conductivity between the negativeelectrode 22 and the positive electrode 21 can be determined by animpedance measurement while an electrolytic solution is presenttherebetween. The permselective membrane desirably has chemicalstability in a solution having a pH of 3 to 10 and heat resistance so asnot to cause degradation or the like at −20° C. to 120° C. Thissuppresses degradation or fracture of the permselective membrane in thesolution, which allows power generation without problems such as ashort-circuit.

In the above-described enzymatic biofuel cell, the fuel solution 26 isintroduced into the anode-solution section 25 through thefuel-solution-introduction port 27. The fuel solution is also suppliedto the cathode-solution section 24 through the permselective membrane.In this case, since the permselective membrane suppresses permeation ofthe fuel component contained in the fuel solution 26, a fuel solutionhaving a low concentration of the fuel component is introduced into thecathode-solution section 24. In other words, the fuel solution being incontact with the negative electrode 22 has a higher concentration of thefuel component than a fuel solution being in contact with the positiveelectrode 21. At the negative electrode 22, the fuel is decomposed bythe enzyme fixed to the surface of the negative electrode 22. Thus,electrons are extracted and protons (H⁺) are generated. At the positiveelectrode 21, water is produced from the protons transported from thenegative electrode 22 through the proton conductor, the electronstransported from the negative electrode 22 through an external circuit,and oxygen contained, for example, in the solution (liquid phase) storedin the cathode-solution section 24 or in the air (gas phase). Since theabove-described enzymatic biofuel cell includes the permselectivemembrane interposed between the anode-solution section 25 and thecathode-solution section 24, which suppresses permeation of the fuelcomponent, diffusion of the fuel component to the positive-electrode-21side may be suppressed. Therefore, even when the concentration of thefuel component in the fuel solution 26 introduced into theanode-solution section 25 is increased, the concentration of the fuelcomponent contained in the solution introduced into the cathode-solutionsection 24 may be kept low, which results in suppression ofdeterioration of the properties of the positive electrode 21.Furthermore, the concentration of the fuel component contained in thesolution being in contact with the negative electrode 22 may be kepthigh, which increases the efficiency of power generation. As a result,an enzymatic biofuel cell that has a battery output greater than orequal to those of the enzymatic biofuel cells known in the related artand that has a larger battery capacity than the enzymatic biofuel cellsknown in the related art may be realized.

Alternatively, as shown in FIG. 16, the immersion-type fuel cell mayhave a structure in which a positive electrode 21 is in contact with asolution 28 containing an electrolyte and the like and a negativeelectrode 22 is in contact with a fuel solution 26. Optionally, agas-liquid separation membrane 30 may be arranged to be in contact withthe outer surface of the positive electrode 21 so that the positiveelectrode 21 is brought into contact with a gas phase (air) with thegas-liquid separation membrane 30. In another case, the positiveelectrode 21 may be arranged to be in direct contact with the gas phase(air) by making the surface of the positive electrode 21 waterrepellent. Current collectors 21′ and 22′ are attached to the positiveelectrode 21 and the negative electrode 22, respectively. Ananode-solution section 25 and a cathode-solution section 24 are providedin the peripheries of the negative electrode 22 and the positiveelectrode 21, respectively. A permselective membrane 23′ is interposedbetween the positive electrode 21 and the negative electrode 22. Afuel-solution-introduction port 27 communicated with the anode-solutionsection 25 and a solution-introduction port 29 communicated with thecathode-solution section 24 are separately disposed. The fuel solution26 is introduced into the anode-solution section 25 through thefuel-solution-introduction port 27. The solution 28 containing anelectrolyte, which is a solution different from the fuel solution 26, isintroduced into the cathode-solution section 24 through thesolution-introduction port 29.

Examples of the solution 28 introduced into the cathode-solution section24 include, but are not limited to, an aqueous solution (electrolyticsolution) containing an electrolyte such as dihydrogen phosphate or animidazole compound; an aqueous potassium chloride solution; and ionicliquid. The solution 28 serves mainly as a proton conductor.

In this enzymatic biofuel cell, the fuel solution 26 is introduced intothe anode-solution section 25 through the fuel-solution-introductionport 27, and the solution 28 such as an electrolytic solution isintroduced into the cathode-solution section 24 through thesolution-introduction port 29. The fuel component contained in the fuelsolution 26 stored in the anode-solution section 25 transfers into thesolution 28 contained in the cathode-solution section 24. Since thepermselective membrane 23′ suppresses permeation of the fuel component,the concentration of the fuel component in the periphery of the positiveelectrode 21 is kept lower than in the periphery of the negativeelectrode 22. The osmotic pressure of the solution 28 is desirably sethigher than that of the fuel solution 26 by, for example, setting theion concentration of the solution 28 introduced into thecathode-solution section 24 to be higher than that of the fuel solution26. This further reduces the amount of fuel component transferred fromthe fuel solution 26 by permeating the permselective membrane 23′. Inthis enzymatic biofuel cell, at the negative electrode 22, the fuel isdecomposed by an enzyme fixed to the surface of the negative electrode22. Thus, electrons are extracted and protons (H⁺) are generated. At thepositive electrode 21, water is produced from the protons transportedfrom the negative electrode 22 through the proton conductor, theelectrons transported from the negative electrode 22 through an externalcircuit, and oxygen contained, for example, in the solution 28 stored inthe cathode-solution section 24 or in the gas phase (air) being incontact with the current collector 21′ via the gas-liquid separationmembrane 30 interposed between the gas phase and the current collector21′.

In this enzymatic biofuel cell, the solution-introduction port 29 iscommunicated with the cathode-solution section 24 and disposedseparately from the fuel-solution-introduction port 27. Therefore,different solutions can be introduced into the anode-solution section 25and the cathode-solution section 24. In addition, the permselectivemembrane 23′ is interposed between the anode-solution section 25 and thecathode-solution section 24. Therefore, even when the concentration ofthe fuel component contained in the fuel solution 26 introduced into theanode-solution section 25 is increased, the amount of the fuel componenttransferred into the solution 28 introduced into the cathode-solutionsection 24 may be kept low. As a result, the concentration of the fuelcomponent in the periphery of the positive electrode 21 may be kept low,which results in suppression of deterioration of the properties of thepositive electrode 21.

The preferred embodiments of the present disclosure are described abovewith reference to examples. The present disclosure is not limited tothese embodiments, and various modifications may be made. Although chaffis used as the raw material of the porous carbon material inabove-described examples, another plant may be used as the raw material.Examples of the other plant include straw, reeds, or wakame stems,vascular plants living on land, pteridophytes, bryophytes, algae, andseaweeds. These materials may be used alone or in a mixture of two ormore. Specifically, for example, rice straw (e.g., straw of rice“Isehikari” produced in Kagoshima prefecture, Japan) may be used as aplant-derived material, that is, the raw material of the porous carbonmaterial. In this case, the straw (raw material) is carbonized to beconverted into a carbonaceous substance (precursor of a porous carbonmaterial), and the carbonaceous substance is then treated with an acidto produce a porous carbon material. In another case, reeds, which arepoaceous plants, may be used as a plant-derived material, that is, theraw material of the porous carbon material. In this case, the reeds (rawmaterial) are carbonized to be converted into a carbonaceous substance(precursor of a porous carbon material), and the carbonaceous substanceis then treated with an acid to produce a porous carbon material. Aporous carbon material produced by being treated using an alkali such asan aqueous sodium hydroxide solution instead of an aqueous hydrofluoricacid solution has the similar properties to that of a porous carbonmaterial produced by being treated with an acid.

Alternatively, wakame stems (produced in Sanriku, Iwate prefecture,Japan) may be used as the plant-derived material, that is, the rawmaterial of the porous carbon material. In this case, the wakame stems(raw material) are carbonized to be converted into a carbonaceoussubstance (precursor of a porous carbon material), and the carbonaceoussubstance is then treated with an acid to produce a porous carbonmaterial. Specifically, for example, the wakame stems are carbonized bybeing heated at about 500° C. The wakame stems (raw material) may betreated with an alcohol or the like prior to being heated. A specificexample of a method for treating wakame stems is a method in whichwakame stems are immersed in ethyl alcohol or the like. This reduces themoisture content of the raw material and causes elements other thancarbon and the mineral component that are contained in the final product(i.e., porous carbon material) to be eluted. Through the treatment withan alcohol, the evolution of gas during the carbonization may besuppressed. More specifically, the wakame stems are immersed in ethylalcohol for 48 hours. An ultrasonic treatment is preferably performed inethyl alcohol. The resulting wakame stems are heated at 500° C. for 5hours in a nitrogen gas stream to be carbonized. Thus, a carbide isproduced. Through the above-described preliminary carbonizationtreatment, the amount of tar component that is to be produced in thenext carbonization process may be reduced or may be removed.Subsequently, 10 g of the carbide is placed in a crucible composed ofalumina, heated to 1,000° C. at a rate of temperature rise of 5° C./minin a nitrogen gas stream (10 l/min), and then carbonized at 1,000° C.for 5 hours to be converted into a carbonaceous substance (precursor ofa porous carbon material). Then, the temperature is decreased to a roomtemperature. The nitrogen gas is kept flowing during the carbonizationand cooling. The precursor of the porous carbon material is treated withan acid by being immersed in an aqueous hydrofluoric acid solution (46%by volume) overnight. The resulting material is then washed with waterand ethyl alcohol until the pH of the material reaches 7 and then dried.Thus, a porous carbon material is produced.

According to an embodiment of the present disclosure, the followingelectrodes, electrode materials, and batteries may be provided.

[1]<electrode> An electrode including a plant-derived porous carbonmaterial having an ability to catalyze oxygen reduction.

[2] An electrode based on the electrode described in [1], wherein theporous carbon material is used for oxygen reduction at a pH of 3 or moreand 10 or less.

[3] An electrode based on the electrode described in [1] or [2], whereinthe specific surface area of the porous carbon material is 100 m²/g ormore as measured by the nitrogen BET method and the pore volume of theporous carbon material is 0.2 cm³/g or more as measured by the BJHmethod and 0.1 cm³/g or more as measured by the MP method.

[4] An electrode based on the electrode described in any one of [1] to[3], wherein the oxygen reduction starting potential of the porouscarbon material is more noble than 0.15 V as measured versus a Ag/AgClreference electrode.

[5] An electrode based on the electrode described in any one of [1] to[4], wherein the porous carbon material includes an oxygen reductioncatalyst supported thereon.

[6] An electrode based on the electrode described in [5], wherein theoxygen reduction catalyst is at least one material selected from thegroup consisting of a noble metal, a transition-metal oxide, atransition-metal porphyrin, phthalocyanine, a porphyrin polymer, aphthalocyanine polymer, perovskite, and a product of pyrolysis of acobalt salt using polyacrylonitrile.

[7] An electrode based on the electrode described in any one of [1] to[6], wherein the porous carbon material is supported on a supportingmember.

[8] An electrode based on the electrode described in any one of [1] to[7], wherein the electrode is used as a positive electrode of a battery.

[9]<electrode material> An electrode material including a plant-derivedporous carbon material having an ability to catalyze oxygen reduction.

[10] An electrode material based on the electrode material described in[9], wherein the porous carbon material is used for oxygen reduction ata pH of 3 or more and 10 or less.

[11] An electrode material based on the electrode material described in[9] or [10], wherein the specific surface area of the porous carbonmaterial is 100 m²/g or more as measured by the nitrogen BET method andthe pore volume of the porous carbon material is 0.2 cm³/g or more asmeasured by the BJH method and 0.1 cm³/g or more as measured by the MPmethod.

[12] An electrode material based on the electrode material described inany one of [9] to [11], wherein the oxygen reduction starting potentialof the porous carbon material is more noble than 0.15 V as measuredversus a Ag/AgCl reference electrode.

[13]<battery> A battery including a positive electrode including aplant-derived porous carbon material having an ability to catalyzeoxygen reduction.

[14] A battery based on the battery described in [13], wherein theporous carbon material is used for oxygen reduction at a pH of 3 or moreand 10 or less.

[15] A battery based on the battery described in [13] or [14], whereinthe specific surface area of the porous carbon material is 100 m²/g ormore as measured by the nitrogen BET method and the pore volume of theporous carbon material is 0.2 cm³/g or more as measured by the BJHmethod and 0.1 cm³/g or more as measured by the MP method.

[16] A battery based on the battery described in any one of [13] to[15], wherein the oxygen reduction starting potential of the positiveelectrode is more noble than 0.15 V as measured versus a Ag/AgClreference electrode.

[17] A battery based on the battery described in any one of [13] to[16], wherein the battery includes an electrolytic solution containing abuffer substance.

[18] A battery based on the battery described in [17], wherein thebuffer substance has a pK_(a) of 4 or more and 10 or less.

It should be understood by those skilled in the art that variousmodifications, combinations, sub-combinations and alterations may occurdepending on design requirements and other factors insofar as they arewithin the scope of the appended claims or the equivalents thereof.

What is claimed is:
 1. An electrode comprising a plant-derived porouscarbon material that catalyzes oxygen reduction at the electrode.
 2. Theelectrode according to claim 1, wherein the plant-derived porous carbonmaterial is a carbonized material treated with at least one of an acidand an alkali.
 3. The electrode according to claim 1, wherein: theporous carbon material is used for oxygen reduction at a pH of 3 or moreand 10 or less when generating electric power; a specific surface areaof the porous carbon material is 100 m²/g or more as measured by thenitrogen BET method; and a pore volume of the porous carbon material is0.2 cm³/g or more as measured by the BJH method and 0.1 cm³/g or more asmeasured by the MP method.
 4. The electrode according to claim 1,wherein an oxygen reduction starting potential of the porous carbonmaterial is more noble than 0.15 V as measured versus a Ag/AgClreference electrode.
 5. The electrode according to claim 1, wherein theporous carbon material includes an oxygen reduction catalyst supportedthereon, and wherein the plant-derived porous carbon material and theoxygen rejection catalyst each increase a rate of the oxygen reduction.6. The electrode according to claim 5, wherein the oxygen reductioncatalyst is at least one material selected from the group consisting ofa noble metal, a transition-metal oxide, a transition-metal porphyrin,phthalocyanine, a porphyrin polymer, a phthalocyanine polymer,perovskite, and a product of pyrolysis of a cobalt salt usingpolyacrylonitrile.
 7. The electrode according to claim 1, wherein theporous carbon material is supported on a supporting member, and whereinthe electrode outputs current during the catalysis of the oxygenreduction.
 8. The electrode according to claim 1, wherein the electrodeis used as a positive electrode of a battery that produces an electriccurrent when the plant-derived porous carbon material catalyzes theoxygen reduction at the electrode.
 9. An electrode material comprising aplant-derived porous carbon material that catalyzes oxygen reduction atthe electrode material.
 10. The electrode material according to claim 9,wherein the porous carbon material is used for oxygen reduction inconjunction with an electrolyte having a pH of 3 or more and 10 or less.11. The electrode material according to claim 9, wherein: a specificsurface area of the porous carbon material is 100 m²/g or more asmeasured by the nitrogen BET method; and a pore volume of the porouscarbon material is 0.2 cm³/g or more as measured by the BJH method and0.1 cm³/g or more as measured by the MP method.
 12. The electrodematerial according to claim 9, wherein an oxygen reduction startingpotential of the porous carbon material is more noble than 0.15 V asmeasured versus a Ag/AgCl reference electrode.
 13. A battery comprisinga positive electrode including a plant-derived porous carbon materialthat catalyzes oxygen reduction at the positive electrode.
 14. Thebattery according to claim 13, wherein the porous carbon material isused for oxygen reduction under a pH-neutral condition.
 15. The batteryaccording to claim 13, wherein: a specific surface area of the porouscarbon material is 100 m²/g or more as measured by the nitrogen BETmethod; and a pore volume of the porous carbon material is 0.2 cm³/g ormore as measured by the BJH method and 0.1 cm³/g or more as measured bythe MP method.
 16. The battery according to claim 13, wherein an oxygenreduction starting potential of the positive electrode is more noblethan 0.15 V as measured versus a Ag/AgCl reference electrode.
 17. Thebattery according to claim 13, wherein the battery includes anelectrolytic solution containing a buffer substance.
 18. The batteryaccording to claim 17, wherein the buffer substance has a pK_(a) of 4 ormore and 10 or less.
 19. The battery according to claim 13, wherein thebattery is one of an enzymatic biofuel cell and a microbial fuel cell.